1. Field of the Invention
The present invention relates to high durability adhesives having for their basis stable resorcinol-terminated urea-formaldehyde resins that are cured under neutral or alkaline conditions with an aldehyde curing agent and are suitable for use in exterior wood products.
2. Description of the Prior Art
It is well known that urea-formaldehyde resins may be advantageously used in forming adhesive compounds; one important advantage of urea resins is relatively low cost. However, the urea resin adhesives are strictly limited to applications in which moisture or high temperatures are not present (as they would be in exterior use). The reason for this limitation is that urea-formaldehyde resins are normally cured with an acid catalyst and this curing reaction is reversible in the presence of moisture or heat, i.e. hydrolysis occurs. Thus, urea-formaldehyde adhesives traditionally have been restricted to interior applications where the adhesive is not exposed to water, humidity, steam, excessive temperatures and the like. The restriction of urea-formaldehyde adhesives to interior use has meant that much higher cost adhesive materials, such as resorcinol resins, must be used in exterior applications.
Resorcinol adhesive resins exhibit high durability or water resistance due to extensive cross-linking of their molecules upon curing. Curing of resorcinol resins, moreover, is accomplished under either neutral or alkaline conditions by reacting the resin with a methylene donor such as paraformaldehyde to cross-link the resorcinol. While urea-formaldehyde resins exhibit stability under neutral or alkaline conditions (i.e., stable against hydrolysis), cross-linking of those resins cannot be effected by the mere addition under neutral or alkaline conditions of a methylene donor because the only result of that procedure is an increase in the chain length of the urea-formaldehyde molecule; and curing of urea-formaldehyde resins under acid conditions results, as aforesaid, in cured resins that are hydrolyzable in the presence of acid conditions or hot water.
Conditions that heretofore have been employed in which a urea-formaldehyde and resorcinol are used together have been such as to break down the urea-formaldehyde usefully to provide the resorcinol with a formaldehyde donor as in Rhodes U.S. Pat. No. 2,437,710 wherein, in those instances in which a urea based donor is employed, either high temperatures, (those at least as high as either that at which resorcinol is molten or reflux temperatures of between 70.degree. and 100.degree. C.) or a decomposing acidic catalyst (oxalic acid) is used. Or in another instance, in Golick et al. U.S. Pat. No. 3,336,246, the resorcinol is employed to form a complex with a protein starch and the urea resin to produce a glue that will rapidly bond thin laminates such as paper board and demonstrate an improved water resistance under the limited conditions under which paper can be used. Thus in none of the foregoing were true resorcinol condensates formed as for example in German Auslegeschrift No. 1102394 and in that publication there is of course no teaching of the neutral or alkaline curing to form urea-formaldehyde based adhesives. It is, indeed, reflective of technical sensitivity of adhesives in this field that phenol formaldehyde adhesives are far from equivalent to resorcinol formaldehyde adhesives, the former while cheaper being entirely unsatisfactory for many of the purposes (e.g. exterior plywood made by normal gluing procedures) for which the resorcinol resin is well suited.
The primary object of this invention is therefore the preparation of an adhesive that is stable in the presence of moisture and heat and therefore is suitable for exterior uses and which nevertheless uses as a base or "backbone resin" the relatively inexpensive urea-formaldehyde resin as compared to the more expensive resorcinol formaldehyde resin which solely could be used for such purposes heretofore.
Another object of the invention is to provide a stable exterior grade adhesive that is based upon a urea-formaldehyde resin and that consequently (as do such urea-formaldehyde resins) has in addition to being economical the advantages of being curable by the radio frequency, (and hence deeply penetrating) method of heating and blend suitably with such fillers as walnut flour as has heretofore been suggested, cf. Ayers U.S. Pat. No. 3,429,770 for the typical resins interior-grade use of urea-formaldehyde resins.